Geoscience Reference
In-Depth Information
to resupply this precious gas, and a cartridge to remove the CO
2
accu-
mulating in the air. Don't ask about the toilet facilities.
We were floating some 1500 meters above the seafloor of the Guay-
mas Basin in the Gulf of California. Running through the Gulf of Cali-
fornia is a spreading zone known as the East Pacific Rise, which sepa-
rates the North American plate to the east from the Pacific plate to the
west. The divergence of these plates slowly drives the Baja Peninsula
away from mainland Mexico. This spreading center is somewhat un-
usual because it includes a thick sediment cover of about one kilometer
deposited over millions of years from particles delivered by the once
raging Colorado River.
2
Seawater circulates through hot rocks of the
spreading center, forming hydrothermal fluids that percolate upward
through the sediment and emerge onto the seafloor, precipitating huge
mounds of gypsum (CaSO
4
•2H
2
O) and supplying abundant sulfide to
the local environment.
We begin our descent through the water column. My nose is glued to
the observation port as we pass slowly through the upper illuminated
zone of the ocean.
3
The light disappears into blackness, and I see the
occasional luminescent flash of an unidentified sea creature. Few words
are spoken, but none are necessary as we sink through the darkness to the
sounds of Brahms playing from the cassette tape player supplied by our
pilot. After about one hour,
Alvin
's outside lights are turned on, and I
blink at the most otherworldly sight. Great mounds of
Riftia
tubeworms
rise from the shadows,
4
swaying gently on expansive hills of gypsum
crust. These elegant sea animals, so beautiful in life, have no mouth or
anus and live by cultivating sulide-oxidizing bacteria in their gut.
Riftia
have evolved an elaborate mechanism for transporting both oxygen and
sulfide to the bacteria, which survive by combining these substances. If
we look closely, what appears to be gently fallen snow on the gypsum
crust is actually a population of free-living sulide-oxidizing bacteria of
the genus
Beggiatoa
(
plate 1)
. They extend to the edge of
Alvin
's lights.
Numerous other animals are also seen, and in one way or another, these
are living of the bounty of microbial life supported by the sulfide ema-
nating from the hydrothermal solutions. As a reminder of this, all around
us, we see the effervescence of hot, sulide-rich, hydrothermal waters per-
colating from the accumulating crust. Sulfide feeds the bacteria, which
feed the animals.
